1. Field of the Invention
The present invention relates to a fixing device, usable with an image forming apparatus, for fixing toner images.
2. Description of the Prior Art
In the fixing device with a heating roller for fixing toner image on a bearing member, there has recently been employed a pressure roller without heating means for simpler structure and reduced power consumption.
In such fixing device, however, when the power supply is turned on while both rollers are stopped, the heating roller alone is heated but the pressure roller remains almost at the room temperature so that, at the start of copying operation after the warming-up cycles the heat of the heating roller is reduced not only by the transfer, sheet but also by the pressure roller of a larger heat capacity, whereby the fixing temperature is significantly lowered to result in a defective fixing. Such phenomenon is particularly marked at the first copying operation in the morning.
In order to avoid such drawback there has been proposed a mechanism for continuously rotating the heating roller and pressure roller, a mechanism utilizing an internally heated pressure roller or a combination thereof, which however are associated with the following drawbacks:
(1) Mechanism for continuously rotating the heating roller and pressure roller: PA1 (2) Mechanism utilizing internally heated pressure roller: PA1 (3) Combination of the above-mentioned mechanisms:
Longer warming-up time; and PA2 Shortened service life of rollers, associated with constant noise. PA2 Longer warming-up time; PA2 Increased machine cost; PA2 Loss in power consumption, as the power allotted to the fixing device has to be divided between the fixing roller and pressure roller in such a well balanced manner as to maintain satisfactory fixing, a reasonable warming-up time and to avoid peeling of rubber layer, thus leading to a power consumption larger than in the conventional fixing device; PA2 Danger of peeling of rubber adhesive; PA2 Increase in machine cost because of the necessary temperature control and complicated structure; and PA2 Slow surface temperature rise of the pressure roller not sufficient for the first copying operation for example in the morning. PA2 Though such combination has the advantages of these mechanisms, it also has the drawbacks thereof.